In a numerically controlled machine tool or robot, motors are driven to achieve machine motion along a plurality of movable axes so that the machine position is controlled to a commanded position as precisely as possible. In this case, an error may occur between the commanded position and the actual machine position because of the influence of machine vibrations and frictional force during reversal of the direction of motion. Therefore, the surface being machined, for example, may be damaged. If such a problem occurs, the machine motion trajectory is measured to find the cause of the problem, and various parameters of a controller for controlling the motors are adjusted. A plurality of methods of measuring and displaying the machine motion trajectory are known.
In the method shown in Patent Literature 1, two high-precision steel balls are connected through a displacement gage, and the displacement when the balls are moved such that the relative distance therebetween is held constant (i.e., along an arc) is read. This method is called a ball bar method and is widely used.
Non Patent Literature 1 shows a method of measuring a motion trajectory in a machine tool using a measuring device called a cross grid encoder. In the measurement using the cross grid encoder, a scale including two orthogonally intersecting optical gratings disposed on a glass substrate and a detection head including two light-receiving units disposed so as to be orthogonal to the respective optical gratings of the scale are used to allow the measurement of two-dimensional relative displacement.
With the method shown in Patent Literature 2, three pairs of an air slide and a linear scale are combined orthogonally to allow the measurement of three-dimensional relative displacement. Patent Literature 3 shows a method of measuring the error between a machine position determined expediently from feedback signals and a machine position designated by a control unit.